I. Field of the Invention
This invention relates generally to electrochemical polarographic gas analyzers and, more particularly, to a method for improving the performance characteristics of gaseous phase oxygen analyzers used to measure oxygen concentration in a sample gas stream.
II. Discussion of the Prior Art
Electrochemical gas analyzers have been available for several years which measure oxygen content of a gas or fluid by diffusing oxygen through a semipermeable membrane into an electrolyte layer proximate a cathode electrode. A polarographic voltage is applied relative to an anode spaced therefrom and the resultant depolarizing current is measured by means of an external circuit.
It is found that the accuracy and linearity of the response is heavily dependent upon the spacing between the membrane and the cathode and by the cathode response characteristics.
The electrochemical gas analyzers also depend on the semipermeable membrane covering the cathode to create stability and longevity to the cell's oxygen response. The cathode where oxygen reduction occurs has an output which is a function of active sites. The cathode material, typically either gold or silver, is configured such that the electrolyte can freely permeate its structure. In this regard, the cathode may comprise a mesh structure of gold or silver wires. An active site is defined as the point where the oxygen molecule, the electrolyte and the cathode come together.
Recent advances in Hersch cell type electrochemical gas analyzers have lead to greatly improved response in the order of 500 milliseconds for a 90% change in oxygen concentration. Further action by electronic circuitry design used with the analyzer have allowed for response augmentation to levels in the range of 100 milliseconds for 0-90% change in concentration. These fundamental advances have been largely achieved by using either very thin or porous membranes. These membranes, typically PTFE, allow for very rapid oxygen transfer, yet still achieve the fundamental basics of preventing electrolyte evaporation loss and providing an intrinsic cathode relationship.
The primary element of the galvanic sensor oxygen analyzer is an electrochemical cell consisting of two electrodes in contact with a liquid or semisolid basic electrolyte (usually potassium hydroxide). The cell electrodes are made of dissimilar metals, such as silver and gold. When a gas sample is introduced into the cell, it diffuses through a Teflon membrane. The oxygen in the sample contacts the gold cathode and is chemically reduced to hydroxyl ions. The hydroxyl ions then flow toward the silver anode, where an oxidation reaction occurs with the silver. This oxidation/reduction reaction results in a flow of electrons proportional to the oxygen concentration of the sample. The electron flow (current) is measured by an external metering circuit connected to the cell electrodes. This current is proportional to the rate of consumption of the oxygen and is indicated on a meter as a percentage or parts per million of oxygen in the sample.
The galvanic sensor oxygen analyzer is essentially a battery that produces energy when exposed to oxygen and, hence, is consumed by exposure to oxygen. It is rugged and insensitive to shock and vibration. The cell can be mounted in virtually any position without changing its sensitivity. The sensor can be packaged as a relatively small, self-contained, disposable cell. It can then be used as a fairly inexpensive means of oxygen measurement in small portable devices. The sensor measures percentage or trace levels of oxygen directly. When properly calibrated, it can provide reliable and accurate measurements. Some sensors can be refurbished rather than replaced by replacing the sensor anode.
Galvanic sensors have several major disadvantages. Because they operate on a battery principle, their life expectancy is a function of usage. Furthermore, as these sensors age, they have a tendency to read low due to a loss in sensitivity. For most process control applications, false low oxygen readings can produce dire consequences. As a result, analyzers that use battery-type sensors must be recalibrated on a frequent basis, sometimes as often as once per day, depending on the criticality of the application.
Another major drawback of battery-type sensors, particularly when used for trace oxygen measurements, is their susceptibility to “oxygen shock.” If exposed to a large concentration of oxygen, these sensors can take several hours to recover. The combination of false low readings, frequent recalibrations, susceptibility to oxygen shock and relatively short life span greatly lessens the value of galvanic sensor oxygen analyzers in many critical applications.
One such commercially available oxygen sensor is the UFO-130-2 sensor available from Teledyne Analytical Instruments, Inc. located in City of Industry, Calif. Because of the porosity and relative thinness of the membrane employed in its construction, it is lacking in tensile strength, such that considerable attention must be paid to the manner in which the membrane is applied to the cathode. Any displacement in the membrane with respect to the cathode can cause the oxygen migration time to increase and an accompanying tendency for oxygen to go into solution in the potassium hydroxide electrolyte utilized in this Teledyne sensor. This affects the time response adversely and in some cases the output stability of the unit is compromised.
The design of the sample gas interface involves an inlet tube and an outlet tube arranged to provide a radial flow over the face of the membrane. Here, volumes are purposely kept extremely low and some flow is necessary to promote good washout and subsequent rapid sensor response to changing oxygen values. To achieve such flow, the gas sample is drawn through the gas sample interface by drawing a partial vacuum. Those skilled in the art will appreciate that the higher the flow, the greater the vacuum that must be applied in the sample chamber. Under these conditions, there is a tendency for the membrane to “lift off” the cathode unless it is well secured. However, there are always limits to the degree of containment, given the dead volume necessary. Under a severe vacuum, the membrane may rupture, allowing loss of electrolyte, followed by a rapid failure of the sensor itself. In less severe circumstances where no rupture occurs, the integrity of the membrane-to-cathode contact may still be threatened, giving rise to a change in response. It is found that small change in the natural response may result in major errors in systems where response is deemed to be a constant. This effect is even amplified when response augmentation electronics are being utilized.
The present invention provides a method for enhancing the performance of a galvanic fuel cell type oxygen sensor, such as the Teledyne UFO-130-2 oxygen sensor. Utilizing the method of the present invention, the pressure drop across the membrane can be reduced thereby minimizing adverse membrane effects without an attendant adverse impact on sensor response. A key feature of the improvement is the application of an equal or similar vacuum to the outside of the pliable electrolyte sac. In addition to protecting the membrane from rupture, the effect of normal atmospheric pressure changes is also negated.